This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. HIV-1 protease and HCV NS3 protease are both enzymes that are required for HIV and HCV infection, respectably. Therefore, these enzymes are considered ideal targets for anti-viral drugs for the treatment of these diseases. For HIV, there are currently 9 protease inhibitors available, and several HCV NS3 protease inhibitors are currently in clinical trials. Unfortunately, drug-resistance to most of these inhibitors has been observed in replicon studies and/or treated patent populations. Our lab views drug-resistance as a change in molecular recognition such that the target enzyme retains the ability to cleave viral substrates but is no longer able to bind inhibitor. In many HIV-1 protease variants multiple site mutations co-evolve to both decrease the affinity of a particular inhibitor and increase the viability and fitness of the enzyme. Crystallographic studies of HIV-1 protease and HCV NS3 protease in complex with substrates and inhibitors will prove to be valuable in order to determine molecular interactions that mediate substrate recognition while decreasing affinity to a particular inhibitor. Structural insights gained by analyzing high resolution crystal structures of HIV-1 protease and HCV NS3 protease in complex with substrates and inhibitors will serve as a guide to develop high affinity inhibitors that are more robust against emerging drug resistant in both viral systems.